The 737Gs are not the 737-6,7,8, & 9 series. They are a glass cockpit 737-300. DAL has both 737-300 and 737-300G series aircraft. The 737-300 aircraft came from Western Airlines and fly out of SLC. The 737-300G came from a few different sources and fly out of ATL. The 737G's have not performed well and have been maintenance intensive. DAL has stated their intention to stop flying this small category soon (9 aircraft?).
I believe the confusion comes from calling the new series 737-6,7,8,9 the 737G. At DAL that designation belongs to the glass cockpit 737-300. And I believe Boeing calls that aircraft the 737-300G as well.
DAL Express flys only 737-200's. They are the very newest of the 737-200 series and are very young. They are flying very few cycles on DAL Express long legs and DAL says it expects to be able to fly them 20 more years. Yes, that would have them retired in their mid 30's. Just about where DAL's 727's, L10's, and DC-9s retired at. And the 727's and DC-9's are flying and will keep flying with other carriers.
The Pilot's contract states that only 737-200's and 737-700's can be used at express pay rates. Other aircraft can be used on express routes but only at mainline payrates and workrules.